Empirical tests of a model of semantic category representation, developed by Caplan in collaboration with R. A. Barr from NIA, have revealed predicted inter-category differences in category structure and inter- category relations. In the model, category representations are described on two "dimensions": 1) whether the features that determine category membership are true of an object in isolation or describe a relation between that object and other object(s), and 2) whether the features are judged to be necessary for all category members, or are only true of some members.